Analysis of the heart rhythm is made based upon electrogram (EGM) signals, collected by electrodes mounted on endocardial leads, implanted in the myocardium. From the EGM, one can measure the atrial and/or ventricular depolarization potential. These signals are then analyzed by the implantable device (IMD), which will, when appropriate, deliver to the patient an appropriate therapy. The therapy delivered may be in the form of, for example, low energy pulses (anti-bradycardia pacing or ventricular resynchronization pacing) or cardioversion or defibrillation shocks.
The implanted leads that are equipped with the electrodes usually have a very thin diameter and a high flexibility, so as to withstand the permanent mechanical constraints to which they are subjected, at the rhythm of heart beats. However, it may sometimes happen that, in spite of their mechanical characteristics, these leads present, over time, a degradation of their external insulation likely to affect sensing of the signal.
Thus, as an estimation, around 10% of the patients implanted with a defibrillator present fractures of the insulating material, or of the conductor, likely to affect sensing of the intrinsic heart rhythm.
These various types of degradations will hereinafter be referred to as a “fracture”, including both fractures as such, (i.e., a total fracture) and incipient fractures or cracks appearing at the very early steps of such phenomena. Indeed, the lead fracture may appear in a very progressive manner, first by a cracking of the insulating material: the electrochemical potentials at the location of the rupture are then likely to disturb the signal of heart rhythm sensing, such disturbance being likely to be erroneously analyzed as a true depolarization of the ventricle.
This phenomenon is every bit as much pernicious as it often appears in a vary progressive manner, by disturbing only a few cycles in the beginning; moreover, if this phenomenon is synchronous with the contraction, it may be blanked over a relatively long duration, or even remain unobserved during patient follow-up examination performed by an electrophysiologist who directly analyzes, in real-time, the signals delivered by the IMD using an external programmer. Also, apart from having an intermittent character, a fracture may affect the ground conductor, in such a manner that it cannot be immediately detected when using a bipolar lead. It is then only once the fracture produces total rupture that the device will detect it, due to the absence of any input signal.
In the meantime, the risks of ventricular oversensing may mislead the device, with a consequent risk that it will lead to inappropriate therapies, for example, by wrongly inhibiting the anti-bradycardia pacing pulses or resynchronization therapies or, conversely, by wrongly delivering high energy shocks upon an erroneous diagnosis of a tachycardia or fibrillation, such shocks being particularly painful for the patient, and likely to be noxious.